Electrolytic pinacol production



1* 2 1946- c. R. BOE1TGE R, JR, ErAL 2,408,036

I ELECTROLYTIC PINACOL PRODUCTION Filed Jul y 11, 1942 r test d sep a, 1946 accents ELEGTRGLYTIC PINACWL ERUQUUEIIQN rice lit. oettger, .llrn, Roselle, Thomas S. (fibers, Newark, and wber G. fiictterbech, mahway, N. St, assignors to Standard (iii Heveiopment @ompany, a corporation oi Deiaware Application July 11, 19%, Serial No. 450,513

' c eas (oi. soc-tr) vide the art with a novel and highly advantageous method of electrolytically forming vicinal glycols and especially pinacols from the corresponding iretones, and also to provide the art with novel electrodes for effecting the reduction of organic compounds.

The electrolytic reduction of ketones can take place at the cathode surface according toany .one of the followingireactionsz C C m 013 (69s.

CHOH Other side reactions result in the formation of metal alky1s,. especially when the reduction is effected in acid medium with lead cathodes.

An electrolytic method for effecting the reductive condensation reaction whereby acetone is converted to pinacol (tetramethylethylene glycol) ,using a lead cathode and an acid catholyte is described in German Patent 113,719 (1899).

Several German patents pertaining to the electrolytic reduction of acetone to tetramethylethylenie glycol were issued in the period 1912-1920. Among these patents were D. R. P. 306,304 (1917), 306,523 (1918) and 324,919 (1920). The principal developments made in this period appear to have been the use of mixed metal and alloy cathodes, such as 4 to copper-96 to 90% lead mixtures converting acetone to pinacol have been unsatisfactory either because pinaool current eficiency in the overall current eficienc or both were too It has now been found that the formation or toxic lead alkyls which diminish the efiectiveness of the cathode can be minimized and the reduction of iretones to vicinal glycols e'fiected with good current eflclency and high ratio otglycols to other reduction products hy utilizing cathodes,

. the surfaces of which contain about 30-80% of and 10% tin-90% lead alloy and the use of horizontally suspended cathodes to prevent the metal 9 alkyls formed from accumulating on and decreasing the activity of the electrode surface.

All of (the prior, processes for electrolytically copper or other metal of similar low hydrogen over-voltage and about -20% of lead.

The cathodes in accordance with the present invention may he'prepared in various ways. The simplest procedure would involve merely mixing finely divided lead and copper in the desired pr0- portion and compressing the mixture into sheets of the desired dimensions. Alternatively the cathodes could be prepared by coating lead particles with copper and subjecting a mass of such coated particles to high pressure and possibly to a heating or sintering treatment. This procedure yields a copper matrix in which lead particles are evenly distributed. By" reversing the procedure and using copper particles coated with lead it is.

powder, screening, or the like, one may also use other metals of low hydrogen overvoltage or alloys such as nickel, brass or Monel metal.

The ketones which may be treated in accord-- ance with the present invention correspond to the general formula wherein the R stands for a member of the group consisting of alkyl and aryl radicals and alkyl and aryl radicals substituted by a group which is not reducible under the conditions employed 1.. e., halogen, carboxyl, etc. Such ketones include, for

ethyl ketone to form the glycol corresponding to th formula cm on. (ls/tn t... n

or methyl ethyl ketone may be condensed for example with-acetophenone to form the glycol The glycols formed in accordance with the" present invention correspond to the general formula:

I wherein each R. stands for an alkyl or aryl radical or an alkyl or aryl radical substituted by a substituent such as halogen, carboxyl; etc.

' A cell-suitable for carrying out the reduction of ketones in accordance with the present invention is shown diagrammatically in the accom panying drawing. The single unit cell shown consists of a rectangular jar or the like 22 arranged in a cooling bath a. An anode e of lead or the like is arranged inside an acid resistant diaphragm d, which divides the cell into an anode and cathode compartment. As shown, two

cathodes c of the composition indicated above are arranged in the cell.

The details of the cell are not critical to our nvention and the cell may be altered in numerous ways. For example, anumber of cells could be combined in a single tank and if desired the process could be carried out continuously by providing a circulating pump, an overflow means for drawing off the electrolyte from the cell, means for separating electrolysis products from the wi drawn catholyte and means for making the electrolyte up to initial strength for re-introduction into the cell.

The use of a diaphragm is recommended since the products formed at the cathode are liable to be oxidized at the anode. The use of a diaphragm may be avoided if desired by the use of anolyte and catholyte of unequal densities in cells provided with horizontal electrodes, the use of a high anodic current density or by the use of anodes having a low oxygen overvoltage, i. e., nickel or by a combination of two or more of these expedients.

Aside from the nature of the cathode surface. the factor which has the greatest effect upon the course of the reaction in the cell is the concentratlon and type of electrolyte used. Acid media have given the best results. The ratio of ketone to water in the catholyte should be high. However, a ratio of 4/1 appears to be approximately the upper limit economically because of 4 catholyte is one containing about 4 volumes of acetone for each volume of sulfuric acid of about 20 per cent strength. The acid, however, may vary in strength between about 10 per cent and about 40 per cent.

The anolyte may be a sulfuric acid solution of about 10' to'40 per centstrength. The electrolyte is preferably maintained at temperatures between about 0 C. and about C. The current density applied may vary between about 0.1 and about 4 amperes/sq. dm., the preferred current density being within the 2 amperes/sq. dm.

' The following examples serve to illustrate our invention but it is to be understood that our invention is not limited thereto.

7 Example 1 576 cc. of a 4/1 acetone-10 per cent sulfuric acid solution were placed in the cathode com partment of a cell as shown in the drawing. The anolyte used was 10 per cent sulfuric acid and the anode was lead. The cathodes used, contained 60% copper and 40% lead.

Electrolysis was eifected at about 16 C. at a current density of 1.67 amps/sq. dm. and at 5.4 volts.

the high resistivity of the solution which would greatly increase the power cost. In the produc-- isopropyl alcohol was 1/1.

The run was continued for 4 hours and a total of 11.2 ampere hours were supplied to the cell. Upon conclusion of the electrolysis the catholyte was stripped of isopropyl alcohol and unreacted acetone and pinacol' (tetramethvlethylene glycol) was separated as the hexahydrate I by cooling and filtering the stripped catholyte. 8.0 grams of pinacol hydrate and 2 grams of isopropyl alcohol were obtained corresponding to a current efilciency based upon pinacolof 17%. The mole ratio of pinacol to isopropyl alcohol was 1/1. l

1 Example 2 3430 cc. of" 4/1 acetone-10 per cent sulfuric acid solution were placed in the cathode compartment of a single unit cell as shown in the drawing. .The anolyte'used was 10 per cent sulfuric acid and the anode was lead. The cathodes used were prepared by rolling copper gauze under pressure onto lead sheets. Rough measurement indicated that the surface consisted of about 70% of copper and 30% of lead.

The electrolysis was effected at about 16 C. at a current density of about 0.84 amp/sq. dm. and at 6.2 to 7 volts. The run was continued for 8 hours and a total of 40.2 ampere hours were supplied to the cell. Upon conclusion of the electrolysis the catholyte was treatedas described in Example 1 to'separate the pinacol hydrate. 42.9 gramsof pinacol hydrate and 11.2v grams of isopropyl alcohol were obtained corresponding .to a

current efiiciency based upon inacol of 25.3% and a current efllciency based upon isopropyl alcohol of 25%. The mole ratio of pinacol to Ewample 3 2340 cc. of a 4/1 acetone-25 per cent sulfuric acid solution were treated in a cell similar to that used in the foregoing examples. The anolyte was 25 per cent sulfuric acid and the anode was lead. 5

The cathodes were prepared bycompressing 20 mesh copper wirescreen into a lead sheet. Rough measurements gave about 40% lead and 60% I copper on the exposed surface of the cathode.

The electrolysis was effected at about 16 C. and a current density of about 1.67 am./sq. dm. and at 5.8 to 6.8 volts. The run was continued for 5 hours. v

range of about 1.5 to about A comparison run was made in. a similar cell with the same volume and strength of catholyte and with a lead anode. The cathode used was a mixture containing 4% copper and 96% lead.

The electrolysis was effected at about 16 C. at

a current density of 2.0 amps/sq. dm. and at 6.3 to 7.8 volts.

The results of both of the foregoing runs are summarized in the following table:

duction of an aqueous'sulfuric acid solution of a ketone of the formula Pmmlhydrm PinacolO a Iso ro 11110 c E 1101mm Time, (grams) p w pinaco hrs. i isglpropyl Total Interval Total Interval Total Interval Total Interval 2 20 21.3 21.3 5.2 :12 25.2 25.2 23.5 n5 1.08/1 50% Cu 40% Pb 4 40 40.0 18.7 8.9 11.1 23.1 222 19.0 1115 1.2 1 5 50 56.5 18.5 11.5 as ace 80.4 20.8 23.2 1.3 1 2 24 0.3 as 0.2 as 4 Cu96%1b 4 48 11.5 11.2 8.7 11.1 1

' c i v 12 28.3 10.8 15.8 as 10.0 10.5 0.41 1

1 Readings and analysis were made at the stated time intervals given.

No lead alkyls were stormed with the cathode having the copper screening rolled thereon but large amounts of lead alkyls were formed and coated the surface of the 4% copper, 96% lead cathode.

" We claim:

1. In the process of producing glycols corresponding to the general formula wherein R stands for a member of the Broup.

wherein R stands for a member of the group consisting of alkyl and aryl radicals and alkyl and.

aryl radicals substituted by a group which is not reducible under the reaction conditions employed the improvement which comprises effecting said reduction in a diaphragm cell in contact with a cathode consisting of from 60 to 80% of copper and to 20% of lead at a temperature below about 25? C. and at a current density of between about 0.1 and about 4 amperes per sq. dm.

2. In the process of producing glycols corresponding to the general formula wherein R stands for a member of the group consisting of alkyl and aryl radicals and substituted alkyl and aryl radicals. by the electrolytic reis not reducible under the conditions employed the improvement which comprises effecting said reduction in a diaphragm cell in contact with a cathode consisting of from 60 to 80% of copper and from 40 to 20% of lead at at temperature below about 25 C. and at a current density of between about 0.1 and about 4 amperes per sq. dm.

3. The process as defined in claim 2 wherein the ketone and sulfuric acid are used in the ratio of about 4 volumes of ketone to 1 volume of acid.

25 C. and at a current density of between about 0.1 and about 4 amperes per sq. dm.

5. .In the process of preparing tetramethy ethylene glycol by the electrolytic reduction of an aqueous sulfuric acid solution of acetone the improvement which comprises eflecting the reduction in a diaphragm cell in contact with a cathode consisting of from about 60 to 80% 0! solution of about 4 volumes of acetone in about ,one volume of aqueous sulfuric acid of about 20% copper and 40 to 20% 0! lead at a temperature below about 25 C. and at a current density of between about 0.1 and about 4 amperes per sq. dm.

6. In the process of preparing tetramethyl ethylene glycol by the electrolytic reduction of a strength the improvement which comprises effecting the reduction in a diaphragm cell in contact with a cathode consisting'of from about to of copper and 40 to 26% oi lead at a temperature below about 25 C. and at a current density of between about 0.1and about 4 ampere:

per sq. dm.

CHARLES R. BOET'IGER, J R 

